1. Field of the Invention
This invention relates to direct casting of metal sheet or strip in a continuous operation and more particularly to an improved method of and apparatus for accelerated casting of thin metal strip by withdrawing the strip in a continuous operation from a supply of molten metal on a chilled casting surface.
2. Prior Art
The conventional practice of forming thin metal sheet by initially casting the metal as an ingot, a thick slab or a plate and subsequently rolling the cast metal into an elongated strip of the desired thin gauge is an expensive and time consuming operation. Accordingly, substantial effort has been directed in recent years to developing a process for directly casting molten metal into a continuous thin strip. One system which has been developed and which is now in commercial use for the production of aluminum sheet casts the metal as a continuous strip on the outside surface of a chilled, driven cylindrical drum. A layer of the molten metal is typically delivered to the chilled casting surface by means of a tundish which is open at one end with the open end being positioned closely adjacent to and mating with the cylindrical casting surface. A tundish and driven cylindrical casting wheel suitable for use in such a strip casting process is disclosed in copending U.S. application Ser. No. 07/179,536, now Pat. No. 4,828,012, the disclosure of which is incorporated herein by reference. In such a system, it is critical to the production of strip of uniform thickness and surface characteristics that the molten metal flowing through the tundish be presented to the chill surface at a substantially uniform temperature across the full width of the strip to be cast. The term "strip" as used herein is intended to mean relatively wide sheet metal in a continuous running length but not to include the narrow continuing ribbon such as is frequently formed in the continuous casting of amorphous metal ribbon which may be only two or three centimeters in width.
A cylindrical chill casting wheel used in the system just described may have an axial length sufficient to produce any desired width of strip within the limits of the apparatus. It has been the practice, however, to utilize a tundish having a transverse width at its outlet or open end which is equal to the desired width of strip to be cast during any particular run. This enables the design and location of baffles, submerged weirs, diffusers and the like to produce the desired uniform flow rate and temperature at the tundish outlet as described in the above-identified copending application. This makes it necessary to stop the process, remove the existing tundish and install a new tundish when it is desired to cast a strip of a different width. This is not only a time consuming operation, but also requires a separate tundish for every width which may be cast, with each tundish being designed and constructed to produce the desired uniform flow. Further, interrupting the casting process results in substantial waste material both as a result of material remaining in the tundish at the end of a run and at the next startup. In addition, and depending upon the length of down time, this procedure may require surface conditioning and maintenance of the cylindrical casting surface.
U.S. Pat. No. 4,751,957 discloses a continuous strip casting apparatus including an air knife employed to assist in the control of strip thickness. Again, a tundish of fixed dimensions is used so that no provisions are made for changing strip width during operation.
U.S. Pat. Nos. 4,715,428 and 4,749,024 and European published application No. 0147912 disclose further tundish configurations for use in direct strip casting. None of these tundish designs provide any means for changing the width of the strip being cast during the casting operation.